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Reflected Heat (and light) question

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Posted by rob on September 18, 2003, 1:51 am
 
Can anyone explain why reflected heat (and light) is more intense than
the direct heat(and light) from the sun?????We were building a solar
air heater and as we were installing the furnace filter material the
heat reflected from the FLAT aluminium reflecter was UNBEARABLE....We
also noticed that the reflected light was stronger than direct
sunlight......The reflecter is constructed from styrofoam house siding
insulation that has a thin alumiunum film on one side...

Thanks
ROB

Posted by M Russon on September 17, 2003, 2:50 pm
 
On 17 Sep 2003 18:51:29 -0700, rcam001@hotmail.com (rob) wrote:


  What are some of the specifics of the solar air heater your
building? I am making a few here myself, and want to learn what others
are doing. What size etc. Thanks in advance.

M Russon

Posted by Eric Jacobsen on September 19, 2003, 8:51 pm
 

I've been thinking about doing something like this for my home in Salt Lake
City.  How important to you think it is to insulate the box?  I have a lot
of old single-pane windows that I was thinking of using.

If I use single-pane windows, do you think it's worth insulating the back?

I like your idea of spray-painted furnace filter suspended with twine.
You'll have to let us know if your hot air smells like paint when it really
gets cooking...



Posted by David Delaney on September 22, 2003, 9:02 pm
 Rob

Styrofoam is NOT SAFE in the interior of your fan dependent solar air
heater. If your fans fail on a sunny day you could get toxic fumes.
STyrofoam is NOT ALLOWED in architectical applications in which its
temperature may exceed 75C (167F). You could easily get 150C(300F), or
even more, inside your air heater when the fans fail, especially since
your glazing consists of thermopane windows.

Polyisocyanurate foam (brand name Thermax) also comes backed with
reflective foil and would be much better, (rated to >300F) but still
only marginally safe. Fiberglass duct board, would be best for the
interior of an active collector whose fans might fail.

As for the heat you feel in front of the aluminum filter material in
the sun, here's what I think may be happening.  Have you ever picked
up a shiny stainless steel tool that has lain in the sun for a while?
It will be surprizingly hot. The same is true for aluminum abjects.
Although these metals reflect most of the sun's energy, they  still
absorb a small fraction of it. Reflective metals are usually just as
reflective at thermal wavelengths as they are at visual wavelengths,
which means that their thermal emmissivity is very low (emissivity +
reflectivity = 1 for opaque surfaces), meaning they lose heat by
radiation quite slowly, and can get quite hot in the sun. So, if you
are standing next to a large well illuminated expanse of such metal,
you get two effects. First, you can get twice as much visual light
energy (direct sun + reflected sun). Second,you are standing near a
large expanse of hot material which, even though it is radiating
thermally at say 0.1 times the power that it would if it were black,
is radiating the spectrum of a high termperature object, and there is
a lot of radiated energy because the filter is big.

David Delaney, Ottawa

On 18 Sep 2003 18:05:49 -0700, rcam001@hotmail.com (rob) wrote:



Posted by David Delaney on September 23, 2003, 11:30 pm
 I said "Polyisocyanurate foam (brand name Thermax) also comes backed
with reflective foil and would be much better, (rated to >300F) but
still only marginally safe."

My memory apears to be faulty. I have been searching the web for
polyiso maximum use temperatures and have found no products
recommended for maximim use temperatures > 250F.

On Mon, 22 Sep 2003 21:02:14 GMT, ddelaney@sympatico.ca (David
Delaney) wrote:



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